Its basically the same way as you would in photoshop, just with a few other steps.

1. So here's your pose. Its a circle in this case cause I'm lazy. Make sure you use a solid brush when drawing, not a soft one.

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2. Select the area around the pose and invert as you normally would. (Use the magic wand tool then go under the selection menu.) Make a layer below that and color it a light shade of gray. Do not use any white for anything in this tutorial.

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3. Shade it like you normally would. Again, don't use white.

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4. Do not color until after the pose is saved. You'll have to color in a different program.

5. When you save, save as a PNG.

6. Open up the pose on this website. It makes the white transparent.SET THE FUZZ TO 0%. Then select the background. If you need help with this, ask.